1. Field of the Invention
The invention relates to the field of interfacing a modem to existing telephone circuitry.
2. Prior Art
In the field of data communications there is an increasing demand for attaching modems to existing telephone circuits. As a consequence of this demand there is the corresponding need for providing a modem-telephone interface which is reliable and easy to use.
One such interface commonly used utilizes a coupling from the handset of a telephone to a device which translates the sound signals emanating from the speaker in the handset into appropriate digital signals to be utilized by the modem. Additionally, the device translates the digital signals from the modem which are to be transmitted over the phone lines into an audio signal which is detected by the microphone in the handset. The device which performs this function is commonly referred to as an acoustic coupler. This accoustic coupler device is used by a terminal user dialing up a phone number known to be connected to an auto-answer modem, whereupon the auto-answer circuitry of this modem connects it (the modem at the receiving end of the call) to the telephone line. Upon hearing the carrier transmitted by this remote modem through the handset, the terminal user places the handset into the accoustic coupler device. A microphone in the accoustic coupler device detects the sound of the carrier from the remote modem through the speaker in the handset. The accoustic coupler then converts it into a digital signal which signals the local modem at the terminal user's site that a connection has been made. This local modem completes the circuit with the remote modem by sending its own carrier through the accoustic coupler and then over the phone lines to the remote modem. After this "hand shaking" procedure is completed by the modems, data communications between the two sites may begin. The reliability of this type of interface suffers as the signals coming over the telephone line are first converted to audio signals and then are converted back into digital signals by the acoustic coupling device.
A second type of interface for performing this function eliminates the need for an accoustic coupler by directly coupling a phone line to a modem. This is accomplished by a terminal user who upon hearing the carrier transmitted by the remote modem, engages a switch which breaks the connection between the telephone and handset. The set of wires which had been connected to the handset are instead coupled to a set of wires going into the local modem.
While this interface eliminates the need for an acoustic coupler, once the handset has been disconnected from the telephone circuit it is no longer possible to hear the carrier coming over the wire or to detect data signals which may be present on the wire. Additionally, a special phone having a switch mounted on it which is capable of performing this switching function must be provided.
It is often desirable to be able to listen to data signals through a speaker, while data communications are taking place, as a diagnostic tool. In the past, this function required the user to have a separate speaker which would be connected to the appropriate wire pair at a telephone company junction box as needed. The inconvenience and extra expense resulting are not desirable. Additionally, the recent blossoming of the home computer market has produced a multitude of users who would like to directly connect their modems to telephone circuitry thereby eliminating the accoustic coupling device. However, they do not wish to have a special phone adapted to perform this function.
The invention disclosed herein overcomes both of the foregoing problems.